This invention relates to continuous casting of thin steel strip in a strip caster, particularly a twin roll caster.
In a twin roll caster, molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are internally cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a thin cast strip product, delivered downwardly from the nip between the casting rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together.
The molten metal may be poured from a ladle through a metal delivery system comprised of a tundish and a core nozzle located above the nip, to form a casting pool of molten metal supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
When casting steel strip in a twin roll caster, the thin cast strip leaves the nip at very high temperatures, of the order of 1400° C. If exposed to normal atmosphere, it will suffer very rapid scaling due to oxidation at such high temperatures. A sealed enclosure is therefore provided beneath the casting rolls to receive the hot cast strip, and through which the strip passes away from the strip caster, which contains an atmosphere that inhibits oxidation of the strip. The oxidation inhibiting atmosphere may be created by injecting a non-oxidizing gas, for example, an inert gas such as argon or nitrogen, or combustion exhaust reducing gases. Alternatively, the enclosure may be sealed against ingress of an ambient oxygen-containing atmosphere during operation of the strip caster, and the oxygen content of the atmosphere within the enclosure reduced, during an initial phase of casting, by allowing oxidation of the strip to extract oxygen from the scaled enclosure as disclosed in U.S. Pat. Nos. 5,762,126 and 5,960,855.
The length of the casting campaign has been generally determined in the past by the wear cycle on the core nozzle, tundish and side dams. Multi-ladle sequences can be continued so long as the source of hot metal supplies ladles of molten steel, by use of a turret on which multiple ladles can be transferred to operating position. Therefore, the focus of attention in the casting campaign has been extending the life cycle of the core nozzle, tundish and side dams. When a nozzle, tundish or side dam would wear to the point that it had to be replaced, the casting campaign would have to be stopped, and the worn out component replaced. This would generally require removing unworn components as well since otherwise the length of the next campaign would be limited by the remaining useful life of the worn but not replaced refractory components, with attendant waste of useful life of refractories and increased cost of casting steel. Further, all of the refractory components, both replaced and continued components, would have to be preheated the same as starting the original casting campaign before the next casting could be done. Graphitized alumina, boron nitride and boron nitride-zirconia composites are examples of suitable refractory materials for this purpose. Since the core nozzle, tundish and side dams all have to be preheated to very high temperatures approaching that of the molten steel to withstand contact with the molten steel over long periods, considerable waste of casting time between campaigns resulted. See U.S. Pat. Nos. 5,184,668 and 5,277,243.
The disclosed apparatus and method limits down time in changes of worn refractory components, decreases waste of useful life of refractory components, reduces energy needs in casting, and increases casting capacity of the caster. Useful life of refractories can be increased, and reheating of unreplaced refractory components can be avoided or minimized. The core nozzle must be put in place before the tundish, and conversely the tundish must be removed before core nozzle can be replaced, and both of these refractory components wear independently of each other. Similarly, the side dams wear independently of the core nozzles and tundish, and independently of each other, because the side dams must initially be urged against the ends of the casting rolls under applied forces, and “bedded in” by wear so as to ensure adequate sealing against outflow of molten steel from the casting pool. The forces applied to the side dams may be reduced after an initial bedding-in period, but will always be such that there is significant wear of the side dams throughout the casting operation. For this reason, the core nozzle and tundish in the metal delivery system can have a longer life than the side dams, and can normally continue to be operated through several more ladles of molten steel supplied in a campaign. Nevertheless, the duration of a casting campaign is often determined by the rate of wear of the side dams because tundish and core nozzle, which still have useful life, are often changed when the side dams are changed to increase casting capacity of the caster. No matter which refractory component wears out first, a casting run will need to be terminated to replace the worn out component. Since the cost of thin cast strip production is directly related to the length of the casting time, unworn components in the metal delivery system are generally replaced before the end of their useful life as a precaution to avoid further disruption of the next casting campaign, with attendant waste of useful life of refractory components.
By the disclosed apparatus and method, it is possible to replace in a minimal period of time anyone or more of the refractory components, for example, the core nozzle, tundish and/or side dams, without replacing any of the other refractory components, to avoid the need for reheating the unreplaced refractory components, and in turn, to extend casting campaign lengths, reduce waste of refractory components, and reduce operating costs and increase casting time.
The second tundish and/or second side dam or dams, or portions thereof, are generally preheated and replaced as singular refractory components, and the core nozzle is generally preheated and replaced as a singular or two part refractory component, but in particular embodiments these refractory components may be preheated and replaced in parts or pieces as desired. In any event, the refractory component or portion thereof may be preheated to a temperature near the temperature of molten steel in the casting pool. Typically, the preheat temperature is greater than about 1200° C. The preheating of rapidly transferring of the second core nozzle may be done for at least about 2 hours before transfer to the operating position, the preheating of rapidly transferring of the second tundish may be done for at least about 2 hours before transfer to the operating position, and the preheating of rapidly transferring of the second side dams may be done for at least about 0.5 hours before transfer to the operating position. If only a portion of one of these refractory components is to be replaced, the preheating of that portion of the component will normally be done for the same time period as for the preheating of the entire refractory component unless that portion is such that it can be preheated to the desired preheat temperature in less time. The preheat temperature is also normally the same if more than one core nozzle, one tundish or two side dams is used in the particular embodiment.
An apparatus for producing thin cast strip by continuous casting may be comprised of:
a) a pair of casting rolls having a nip therebetween;
b) a metal delivery system comprising a first core nozzle and a first tundish for delivering molten metal into a casting pool between the casting rolls above the nip, and first side dams adjacent the ends of the nip to confine said casting pool;
c) a casting roll drive capable of counter-rotating the casting rolls to form solidified metal shells on casting surfaces of the casting rolls and to cast solidified thin steel strip through the nip between the casting rolls from said solidified shells;
d) a plurality of preheating chambers removed from an operating position for casting capable of preheating at least a portion of at least one refractory component selected from the group consisting of a second core nozzle, a second tundish and at least one second side dam to a temperature to avoid thermal shock when contacted by molten steel while casting continues;
e) a gate capable of interrupting the flow of molten metal to the casting pool, and capable of resuming flow of molten steel to reform the casting pool;
f) a first transfer device capable of removing from an operating position at least portions of at least one component selected from the group consisting of at least a portion of the first core nozzle, the first tundish and at least one of said first side dams desired to be replaced while leaving other components in an operating position; and
g) a second transfer device capable of rapidly transferring at least portions of at least one preheated component selected from the group consisting of the second core nozzle, the second tundish and at least one second side dam for replacement from the preheating chamber to the operating position for casting while leaving other components in an operating position.
Again, in the apparatus, the second tundish and/or second side dam or dams, or portions thereof, are generally preheated and replaced as singular refractory components, and the core nozzle is generally preheated and replaced as a singular or two part refractory component, but in particular embodiments these refractory components may be preheated and replaced in parts or pieces as desired. In any event, at least one component from the group consisting of the second core nozzle, the second tundish or the second side dams may be preheated to a temperature near the temperature of molten steel in the casting pool. Again, typically the component or components, or portion thereof, to be replaced is/are preheated to 1200° C. The preheating of the second core nozzle may be done for at least about 2 hours before transfer to the operating position, the preheating of rapidly transferring of the second tundish may be done for at least about 2 hours before transfer to the operating position, and the preheating of rapidly transferring of the second side dams may be done for at least about 0.5 hours before transfer to the operating position. Again, if only a portion of one of these refractory components is to be replaced, the preheating of that portion of the refractory component will normally be done for the same time period as for the preheating of the entire refractory component unless that portion is such that it can be preheated to the desired preheat temperature in less time. The preheat temperature is also normally the same if more than one core nozzle, one tundish or two side dams is used in the particular embodiment.
The apparatus may further comprise a sensor, such as an optical sensor or an electrical sensor, to monitor the wear of the first core nozzle, the first tundish and/or the first side dams. The method may further comprise the step of monitoring the wear of at least a portion of one refractory component from the group consisting of the first core nozzle, the first tundish and the first side dams. The first core nozzle, first tundish or first side dams may be removed one at a time, or in pieces, when the sensor reveals that the refractory component is worn to a specified limit. Note again that when a refractory component is replaced in parts as worn, a separate sensor will normally be provided for each piece of the refractory component to be replaced as worn.
The apparatus may also be automated by including in addition a control system, typically including a computerized circuit, so that, when a given level of wear is detected by the sensor(s) in a particular worn first core nozzle, first tundish and/or first side dames), or portion thereof, the worn refractory component or portion thereof is automatically replaced. Note that when a refractory component is replaced in parts as worn, a separate sensor will normally be provided for each portion of the refractory component to be replaced as worn.
Alternatively, the apparatus may have a preheating chamber or chambers removed from an operating position for casting thin cast strip capable of preheating one or both of the second side dams, or portions thereof, to a temperature to avoid thermal shock when contacted by molten steel. In this embodiment, the core nozzle or the tundish, or both, (or a part thereof) may be replaced independently of the side dams. It should be noted that the apparatus can be embodied if more than two side dams are desired to be utilized in a particular embodiment.
The molten steel may be introduced between the casting rolls through a metal delivery system comprising a tundish and a core nozzle, in one or more pieces, disposed above the nip, and the interruption of the flow of molten steel to the casting pool may be achieved by interrupting flow to the metal delivery system by closing the slide gate. The preheating of the replacement side dams in the preheat chamber(s) is initiated while continuing casting of the strip. The wear of the side dams may be monitored by a sensor or sensors, and the removal and replacement of the side dams may be accomplished when the sensor indicates that the dams or portion thereof is (are) worn to specified limits.
In order to ensure the components in the metal delivery system do not suffer thermal shock on resumption of casting and also to ensure that steel does not solidify within the flow passages of the metal delivery system, it is desirable that the time interval between interrupting and resuming the flow of molten steel in either the method or the apparatus be less than about 15 minutes. The change time will depend on the number and nature of the component or components being replaced, and typically will be less than about 5 minutes.
More specifically, the replacement of the replacement one or more side dams, tundish and/or core nozzles, or portions thereof, may be carried out so that this time interval is about 5 minutes or less, or about 2 minutes or less.
It should be noted that the tundish here that is replaced is a replaceable tundish above the core nozzle, and may be sometimes called the transition piece or delivery vessel.
There may be another tundish above the replaceable tundish, which is also part of the metal delivery system that is not replaced in the disclosed method and apparatus as discussed below.